The present invention generally relates to ultrasonic inspection systems utilized to detect internal structural faults, e.g. cracks, discontinuities, corrosion or thickness variations within an object or a material, for example, in such crucial structures as airline wings. This is done by transmitting ultrasonic pulses to a target object and analyzing echo signals detected from the target object. More particularly, the present invention relates to a high dynamic range analog to digital conversion system and method which can be used in such ultrasonic inspection systems, particularly whereby the object is scanned with an ultrasonic probe or transducer. The present invention also relates to eddy current inspection systems utilized to detect internal structural faults.
The prior art of ultrasonic flaw detectors is exemplified by such products as the instant assignee's Epoch 4 Plus product. Competitive products available from General Electric are known as the USM 35X, USN 58L and USN 60 fault detection systems. In general, prior art ultrasonic flaw detectors utilize highly complex analog front ends that contain many parts which pose especially difficult problems in terms of calibration, reliability, set up time, consistency of results and optimization for specific usages and settings.
Typical prior art ultrasonic flaw detectors include a transducer which is placed against the object to be tested and which works in conjunction with numerous analog circuits such as gain calibrators, preamplifiers and attenuators, variable gain amplifiers, and high pass and low pass analog filters that operate over many different frequency bands and which need to be carefully calibrated and maintained.
As a result, present flaw detectors present a host of problems to designers and users of such equipment, which impact their troubleshooting and repair owing to their complexity. These problems include such issues as matching input impedances seen by the transducer which changes with different gain amplifiers that are switched in and out of the signal path. This adversely impacts the frequency response and introduces various gain nonlinearities. It poses issues of calibration, as analog circuits are switched in and out of the signal path.
Another problem with existing flaw detectors is attributable to their back wall attenuation performance which impacts the ability to detect flaws that are located very near the back wall of the object being tested. This problem poses particular problems with the time varied gain function which has a limited gain range and gain rate of change in prior art devices.
Another prior art drawback ensues from the manner in which analog circuits are coupled, which results in each operational amplifier in the signal path having different DC offset errors that must be nulled in order to keep the input signal at the mid-point of the analog to digital converter being utilized, in order to allow the maximum full amplitude scale of such converter to be utilized. Furthermore, the DC offset errors can cause the waveform presented on the display to not be centered vertically on the waveform portion of the screen, thereby causing an undesirable anomaly in the waveform that the operator analyzes to determine the results of their inspection. The error nulling processes in the prior art are therefore unreliable, particularly at high gain, due to DC baseline measurement inaccuracies caused by noise.
The intensely analog implementation of the front ends of existing flaw detectors poses further issues owing to the need to utilize the entire dynamic range of the instrument to be utilized which creates various gain linearity calibration issues.
An ultrasonic inspection apparatus of the prior art is described in U.S. Pat. No. 5,671,154, which provides background information for the apparatus and method of the present invention.